RETURN OF THE HORNWORTS

The text of this page is taken from an article by Anna Levin in the Summer 2003 issue of The Botanics journal, published by the Royal Botanic Gardens, Edinburgh. It is reproduced here by kind permission of the editor. Further details, including an Adobe Acrobat version of the issue can be found on their web page http://www.rbge.org.uk/rbge/web/pub/bot.jsp

The stubble field locality in Berwickshire

On a rainy day last November, RBGE bryologist David Long was trudging through a muddy field in Lauderdale. He’d been down by the riverside collecting samples of mosses and liverworts for the next day’s teaching. Heading home across a stubble field, he spotted a small plant at his feet with flat green rosettes about the size of 50p pieces and long, stalk-like ‘horns’ growing out of them. He immediately recognised it as a hornwort, but it wasn’t until he got it back to the herbarium that he realised it was the Carolina hornwort Phaeoceros carolinianus, an endangered species never recorded in Scotland before.

This chance find was exciting because hornworts, described by David as “puzzling, ancient plants”, are thought to be extremely scarce in Britain, and the Carolina is the rarest, previously only recorded in a few locations in the south of England. It was also a timely find as RBGE scientists had just begun participating in a survey run by the British Bryological Society to record the presence of bryophytes on arable land, and so David was able to return to the site with the survey team. The project arose from concerns that intensive agricultural practices may be damaging this rich habitat for bryophytes. “In the old days, farmers would leave a field fallow after harvest right through to the spring,” explains David. “Bryophytes, which grow in the autumn, thrived there, as did other wildlife – the fallow time allowed flowering plants to set seed, and birds to feed on flower seed and cereal grains. Now stubble fields are ploughed immediately after harvest, and the fear is that these arable bryophytes do not have time to complete their life cycle and may be in severe decline. Of course it’s not just bryophytes that are affected by this change, but it might be that they are a very good indicator of habitat quality.”

RBGE is a leading centre of bryophyte research, with teams engaged in a variety of projects, including cataloging bryophytes in the mountains of Nepal and China, studying the evolution and taxonomy of liverworts, and testing Scottish bryophytes for poten-tially useful pharmaceutical compounds. David Long is RBGE’s Head of Bryology and current president of the British Bryological Society – he’s a dedicated champion of what he considers an undervalued group of plants. “Just because they’re small doesn’t mean they’re not interesting,” he says. “Once you get up close and really look at them, you realise they’re incredibly intricate and beautiful. Their value is subtler than more glamorous plants, but they have an important ecological role. For example, in arable land or sand dunes, they’re pioneers – the first plants to get a foothold and stabilise the soil. “Bryophytes also have a vital role in storing water. In tropical forests or wet woodland, the mossy carpet on the logs and forest floor acts like a sponge, absorbing moisture and slowly releasing it. This ‘bryophyte mat’ also forms an important habitat – insects shelter in it and plants such as ferns and orchids thrive there. “To me as a scientist, one of the most interesting things about bryophytes is that they are so ancient. They appear to have been the first plants to colonise land, and all subsequent evolution of land plants has developed from them. The first records of liverworts date back to the late Devonian period – about 400 million years ago – and they haven’t really changed much since.”

David explains that one of the reasons bryophytes have survived so long is that they have developed chemical defences, so very few things eat them – even deer and rabbits don’t eat mosses in a woodland. “These chemicals stop bacteria attacking them and may be of use in medicine,” he says. “In World War One, RBGE was an important collection centre for Sphagnum moss, which was shipped to the front and used in surgical dressings because of its antiseptic and absorbent properties. But we don’t know enough about the medicinal potential of other bryophytes.” Recent research on the evolution of land plants and advances in DNA studies has led to a greater appreciation of the importance of bryophytes, stimulating more focused research.

One such study is the ‘Survey of the Bryophytes of Arable Land,’ a three-year project which aims to harness the skills and enthusiasm of professional and amateur bryologists throughout the UK. “In Scotland, we’re very lucky to have an extremely rich bryophyte flora,” says David. “But whereas some habitats, such as the Atlantic wood-lands of the west coast, are famous for their mosses and liverworts, others with less intrinsic appeal get neglected. “We decided to make agricultural land a special study area. The idea is to create a baseline – if you want to monitor change you have to have a starting point. We have a feeling that arable bryophytes are in decline, but to prove this we need accurate data.”

Survey teams throughout the country searched specified sites in cereal-growing areas, taking detailed recordings of all bryophytes found, as well as the context of the find, such as the crop on the field and the pH of the soil. Most of the species are tiny and so only careful searching on all fours, followed by microscopic examination of samples back at the herbarium, will reveal all species present in a certain site. But though last winter’s wet weather was bad news for farmers, and made for muddy work for botanists, it seems it was good news for bryophytes, thanks to the lack of ploughing. The discovery of hornworts was the most exciting find of the project, with three of the four British species found in the Borders. The Carolina hornwort has turned up in four fields in Berwickshire and Roxburghshire, growing in damp corners of stubble fields, often close to trees, which give extra shade. The field hornwort and dotted hornwort have also been found in the Borders, though less frequently.

Not much is known about hornworts. They are classed as bryophytes because they superficially resemble liverworts, but exactly how or if they are related is not yet clear. Some believe them to be a unique and ancient branch of early land plants. Their reproduction is certainly unique – while liverworts shed their spores in a single explosive burst, the ‘horns’ are elongated capsules that keep growing and produce spores continuously over several weeks. They also have a unique symbiotic association with blue-green algae that is unknown in other bryophytes. Are these enigmatic plants making a comeback in Scotland? Or are they more widespread than was previously thought? Perhaps they’ve always been there, hidden in the soil – some bryophyte spores can last for 50 years – waiting until the conditions were right. These questions inspire bryologists to get down on their hands and knees and search through stubble fields for these ancient plants. But the survey still has some years to go and the national results must be gathered and analysed before we can answer these questions, and ask many more.